Genetic studies on inherited life span variations in the myxomycete Didymium iridis will be undertaken with the aid of the unique clonal gametes of this diplohaplontic species. The haploid uninucleate amoebal stage grows vegetatively (clonal population from a single cell) and can also serve directly as a gamete. This allows the reuse of identical gametes in a number of crosses at different times and makes the analysis of complex polygenic systems possible. The diploid multinucleate plasmodial stage resulting from a cross of two compatible gametic clones has a characteristic life span (70 to 140 days) which is determined by its genotype. Termination of this life span takes place by means of progressive senescence which is characterized by cell necrosis and nuclear abnormalities. The objective of this research is to rigorously define the polygenic system controlling the life span and to investigate the mechanisms of senescence. This information should prove very useful in evaluating the programmed cell senescence theory of aging.